Twin-roll strip caster, method for manufacturing thin duplex stainless steel sheet using the same and thin duplex stainless steel sheet

ABSTRACT

Provided are a twin-roll strip caster, a method for manufacturing a duplex stainless steel sheet using the same, and a thin duplex stainless steel sheet. The twin-roll strip caster includes a pair of casting rolls rotating in opposite directions, an edge dam installed such that a molten steel pool is formed on respective sides of the casting rolls, and a meniscus shield provided to cover the molten steel pool such that a contact between the molten steel pool and air is blocked, wherein hills and valleys are alternately arranged on surfaces of the cast rolls in circumferential directions thereof and a hill area in edge sections thereof is higher than that in centers sections thereof. Since cast rolls in which a hill area ratio of edge sections is higher than that of center sections are used, a thin stainless steel sheet with improved edge quality may be provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No. 10-2014-0186477 filed on Dec. 22, 2014, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a twin-roll strip caster, a method for manufacturing a duplex stainless steel sheet using the same, and a thin duplex stainless steel sheet.

2. Description of the Related Art

In general, austenitic stainless steels having good workability and corrosion resistance contain iron (Fe) as a matrix metal, and chromium (Cr) and nickel (Ni) as major ingredients, and in this regard, various types of steel have been developed from such austenitic stainless steels by adding other elements such as molybdenum (Mo), copper (Cu) or the like, and since 304 and 316 series stainless steels with good corrosion resistance and workability contain relatively expensive ingredients such as Ni, Mo or the like, 200 series and 400 series stainless steels have increased in popularity as alternatives. However, 200 series and 400 series stainless steels do not have superior characteristics to 300 series stainless steels in terms of formability and corrosion resistance.

Meanwhile, duplex stainless steels in which an austenite phase and a ferrite phase are mixed have both the advantages of austenitic stainless steels and the advantages of the ferritic stainless steels, and thus, various kinds of duplex stainless steels have been developed. Since duplex stainless steels commonly contain a large amount of nitrogen to increase corrosion resistance, duplex stainless steels exhibit superior corrosion resistance in various corrosive environments, as compared with austenitic stainless steels such as 304 series and 316 series stainless steels. However, such duplex stainless steels commonly contain relatively expensive elements such as nickel (Ni), molybdenum (Mo), or the like, and thus, the manufacturing costs thereof may be increased.

To increase the price competitiveness of such duplex stainless steels, interest in lean duplex stainless steels in which relatively expensive alloy elements such as Ni, Mo or the like contained in the duplex stainless steels are excluded and relatively inexpensive alloying elements are added has increased. However, such lean duplex stainless steels have limitations in terms of surface cracks and edge cracks, due to poor hot workability caused by a difference in strength between a ferrite phase and an austenite phase.

PATENT DOCUMENTS

Patent 1: U.S. Pat. No. 5,624,504 entitled ‘Duplex structure stainless steel having high strength and elongation and a process for producing the steel’ published on Apr. 29, 1997

Patent 2: Korean Patent Publication No. 2013-0135575 entitled ‘Method for producing high nitrogen thin duplex stainless steel sheet’ published on Dec. 11, 2013

SUMMARY OF THE INVENTION

An aspect of the disclosure may provide a twin-roll strip caster capable of manufacturing a thin stainless steel sheet having improved edge quality, a method for manufacturing a thin duplex stainless steel sheet using the same, and a thin duplex stainless steel sheet.

According to an aspect of the present invention, there is provided a twin-roll strip caster including: a pair of casting rolls rotating in opposite directions; an edge dam installed such that a molten steel pool is formed on respective sides of the pair of casting rolls; and a meniscus shield provided to cover the molten steel pool such that a contact between the molten steel pool and air is blocked, wherein hills and valleys are alternately arranged in circumferential directions on the pair of casting rolls and a hill area ratio (an area ratio of hills) in edge sections thereof is higher than that in center sections thereof.

The hill area ratio may be constant in the center section and may continuously increase in the edge section in a direction away from a boundary between the edge section and the center section.

The hill area ratio in the center section may be in a range of about 10-40% and the hill area ratio in the edge section may increase up to 70%.

The edge section may have a width of 50-200 mm from one ends of the pair of casting rolls.

A gas discharge index G of the center section may have a value of 80-130 and the gas discharge index G of the edge section may continuously decrease to a range of at least 50-70 from a boundary of the center section, in which G=width (W) of valley depth (d)/pitch (p).

According to another aspect of the preset disclosure, a method for manufacturing a thin duplex stainless steel sheet, the method includes: forming a cast strip by pouring molten steel between a pair of cast rolls rotating in opposite directions; and manufacturing a hot rolled strip by rolling the cast strip in a rolling mill, in which hills and valleys are alternately arranged on surfaces of the pair of cast rolls in circumferential directions thereof and a hill area ratio (an area ratio of hills) in edge sections thereof is higher than that in centers sections thereof.

The reduction ratio may be in a range of 15-60%.

The above method may further include annealing the hot rolled strip in which the annealing temperature maybe in a range of 1,000-1,250° C.

According to another aspect of the present invention, a duplex stainless steel sheet manufactured by the above-described method is provided.

The thin duplex stainless steel sheet may include, by weight: 0.1% or less carbon (C) (exclusive of 0%); 0.2-3.0% silicon (Si); 1.0-4.0% manganese (Mn); 19.0-23.0% chromium (Cr); 0.3-2.5% nickel (Ni); 0.15-0.3% nitrogen (N); 0.3-2.5% copper (Cu); a balance of iron (Fe); and inevitable impurities.

The thin duplex stainless steel sheet may have an elongation of 25-55% in a direction perpendicular to the rolling direction, and a yield strength of 350-700 MPa.

The thin duplex stainless steel sheet may have a recrystallized grain size of about 4-9 μm and a necking-down width of 10 mm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view of a twin-roll strip caster;

FIG. 2 is a schematic view illustrating a surface of a cast roll in a twin-roll strip caster according to an exemplary embodiment of the present invention;

FIG. 3 is a three dimensional image showing a surface of a cast roll in a twin-roll strip caster according to an exemplary embodiment of the present invention;

FIG. 4 is a graph showing a hill area ratio and a gas discharge index in a width direction of a cast roll according to an exemplary embodiment of the present invention;

FIGS. 5A and 5B are high temperature photographs of slabs according to a comparative example and an example of the present invention;

FIGS. 6A and 6B are surface photographs of casting materials according to a comparative example and an example of the present invention; and

FIGS. 7A and 7B are microstructure photographs after cold rolled annealing of casting materials according to a comparative example and an example of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings.

Embodiments of the disclosure may, however, be embodied in many different forms or combined and the scope of the present invention should be construed as being limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will more fully convey the concept of the invention to those skilled in the art. In the drawings, the shapes and sizes of elements may be exaggerated for clarity. Like reference numerals in the drawings denote like elements.

FIG. 1 is a schematic view of a twin-roll strip caster according to an exemplary embodiment of the present invention, and the twin-roll strip caster includes a ladle 1 receiving molten steel, a tundish 2 into which the received molten steel is introduced, a pair of cast rolls 5 rotating in opposite directions, an injection nozzle 3 supplying molten steel to a space between the pair of cast rolls 5, an edge dam 6 forming a sump 4 such that a molten steel pool is formed on respective sides of the pair of cast rolls 5, and a meniscus shield 7 provided to cover an upper side of the molten steel pool and to block contact between the molten steel pool and air. Also, the twin-roll strip caster may further include a rolling mill 8, a cooling machine 9, and a coiling machine 10.

In a twin-roll strip casting process according to an exemplary embodiment of the present invention, molten steel is received in the ladle 1 by using the twin-roll strip caster illustrated in FIG. 1, and the received molten steel is introduced into the tundish 2 through a nozzle. The molten steel introduced into the tundish 2 is supplied to the edge dam 6, i.e., between the cast rolls 5, through the molten steel injection nozzle 3 and starts to be solidified. At this time, the meniscus shield 7 may prevent oxidation of the molten steel in the molten steel pool between the cast rolls 5 at an upper surface of the molten steel pool, and the atmosphere surrounding the molten steel pool may be adjusted by injecting a predetermined gas. The molten steel may be extruded and manufactured into a strip while passing through a roll nib at a point at which the cast rolls 5 meet. Then, the strip is rolled into a thin steel sheet while passing through the rolling mill 8, and the thin steel sheet is cooled while passing through the cooling machine 9 and is wound by the coiling machine 10.

In the twin-roll strip casting process directly manufacturing a strip having a thickness of 10 mm or less from molten steel, it is important that molten steel is supplied through the injection nozzle 3 between the internal cooling type cast rolls 5 rotating in opposite directions at a rapid rate to manufacture the strip at a desired thickness without cracks with an improved actual yield.

Also, in order to manufacture, by using a twin-roll strip casting process, a thin steel sheet such as a high nitrogen-containing duplex stainless steel sheet in which gas exhaust occurs, a cast roll surface treatment technique that enables gas to be exhausted is necessary and a control for uniform cooling in the width direction is required.

Referring to FIG. 2, a portion of a surface 5S of the cast roll 5 in a twin-roll strip caster according to an exemplary embodiment of the present invention is illustrated. The surface 5S of the cast roll 5 may include edge sections having a predetermined width from one end, and center sections between the edges sections, and FIG. 2 illustrates a region including a boundary between the edge section and the center section.

Hills 110 and valleys 120 extending in a casting direction or a rolling direction of the cast roll 5 may be alternately formed on the surface 5S of the cast roll 5. A three dimensional image of a portion of the surface of the cast roll 5 is shown in FIGS. 5A and 5B. That is, the hills 110 and the valleys 120 may be arranged in linear manner in a circumferential direction of the cast roll 5. In the case of high nitrogen duplex stainless steel, since nitrogen gas is exhausted due to a solubility difference during solidification of molten steel, the surface 5S of the cast roll may be processed to have the valleys 120 so that gas may be easily exhausted.

Particularly, according to an exemplary embodiment of the present invention, the area ratio of the hills 110 may decrease in a direction from the edge section toward the center section. Thus, a width (L1) of any one of the hills 110 of the edge section may be wider than a width (L2) of the hill 110 adjacent to the center section. Also, the widths L1 and L2 of the hills 110 of the edge section may be wider than a width L3 of the hill 110 of the center section.

Referring to FIG. 4, a hill area ratio and a gas discharge index Gin a width direction of a cast roll according to an exemplary embodiment of the present invention are illustrated. The graph of FIG. 4 shows variations according to a distance from one end of the edge section, i.e., one end of the cast roll from one end of the edge section toward the center section.

The hill area ratio in the center section may be constant in a range of 10-40% but the embodiments of the present invention are not limited thereto. If the hill area ratio in the center section is less than 10%, the cast roll and a solidification shell maybe adhered to each other to make it difficult to perform a casting operation, and if the area ratio is more than 40%, a solidification ability difference between the center section and the edge section is not significantly high, so that it may be difficult to prevent solidification delay of the edge section.

The hill area ratio in the edge section may be larger than that in the center section. Also, the hill area ratio in the edge section may increase in a direction away from the center section but the embodiments of the present invention are not limited thereto. For example, the hill area ratio in the edge section may be in a range of 10-70% and may be continuously changed. The maximum hill area ratio of 70% in the edge section is a value designed in consideration of gas exhausting.

A transition boundary of the hill area ratio between the edge section and the center section, i.e., a boundary at which the hill area ratio is changed and is then made constant, may be in a range of 50-200 mm from one end of the cast roll. That is, the width of the edge section may be in a range of 50-200 mm from one end of the cast roll. The transition boundary may correspond to a position at which solidification delay occurs along the edge section.

The gas discharge index G of the center section may be in a range of 80-130 and the gas discharge index G of the edge section may be continuously decreased to a minimum range of 50-70. Herein, the gas discharge index G indicates the area of hill per unit pitch and is expressed by the following equation: G=width (w) of valley x depth (d)/pitch (p).

When the gas discharge index G is lower than 80, micro-cracks or depressions may be generated on a surface of a casting material, and when the gas discharge index G is 130 or higher, the depth of valley is so deep that the cast roll and the solidification shell may be adhered to each other to make it difficult to perform the casting.

If a cast roll having a constant hill area ratio along the width direction is used, solidification delays may be generated in the edge section so that edge bulging or the leakage of molten steel may be generated. However, according to an exemplary embodiment of the present invention, the degree of solidification may be controlled by manufacturing high nitrogen lean duplex stainless steel with the cast rolls in which hills and valleys are adjusted as above. It could be understood from experimental results that the higher the hill area ratio, the more the solidification ability is enhanced, and edge bulging was prevented by increasing the hill area ratio of the edge section based on such fact. Also, valleys shaped in fine grooves were formed such that gas discharge index G had a predetermined value or higher, and the valleys were differently applied to the edge section and the center section so that casting materials with good surface and edge qualities may be manufactured.

Hereinafter, a method for manufacturing a thin duplex stainless steel sheet according to an exemplary embodiment of the present invention will be described in more detail.

According to an exemplary embodiment of the preset disclosure, a method for manufacturing a thin duplex stainless steel sheet, the method includes: forming a cast strip by pouring molten steel between a pair of cast rolls rotating in opposite directions; and manufacturing a hot rolled strip by rolling the cast strip in a rolling mill.

Hills and valleys are alternately arranged on surfaces of the pair of cast rolls in circumferential directions thereof and a hill area ratio (i.e., an hill area ratio) in edge sections thereof is higher than that in centers sections thereof.

The cast strip may have a thickness of 1-6 mm and a width of 1,000-1,400 mm.

The reduction ratio in the rolling may be in a range of 15-60%.

If the reduction ratio is less than 15%, pores may be generated in a central segregation section so that product quality may be deteriorated, and if the reduction ratio is more than 60%, rolling may be impossible due to the limitations of specifications of rolling facilities.

The hot rolled strip may have a thickness of 0.7-4 mm and a width of 1,000-1,400 mm.

The above method may further include annealing the hot rolled strip in which the annealing temperature maybe in a range of 1,000-1,250° C.

Hereinafter, a thin duplex stainless steel sheet manufactured according to an exemplary embodiment of the present invention will be described in more detail.

A thin duplex stainless steel sheet according to an exemplary embodiment of the present invention may include, by weight: 0.1% or less carbon (C) (exclusive of 0%), 0.2-3.0% silicon (Si), 1.0-4.0% manganese (Mn), 19.0-23.0% chromium (Cr), 0.3-2.5% nickel (Ni), 0.15-0.3% nitrogen (N), 0.3-2.5% copper (Cu), a balance of iron (Fe), and inevitable impurities. However, minimum amounts of phosphorous (P) and sulfur (S) may be included in order to suppress segregation.

Carbon (C) is an element for forming an austenite phase and is an effective element for increasing strength of a material by solid-solution strengthening. When C is, however, added excessively, C is easily bonded to a carbide-forming element such as chromium (Cr) that is effective for corrosion resistance in a boundary between a ferrite phase and an austenite phase to decrease the content of Cr and the corrosion resistance. Therefore, C may be added in an amount of 0.1% or less to maximize the corrosion resistance.

Silicon (Si) is an element which is partially added to achieve a deoxidizing effect, used to forma ferrite phase, and is concentrated on ferrite during an annealing treatment. Therefore, Si is added in an amount of 0.2% or more to secure an appropriate ferrite phase fraction. However, if Si is added in an amount of 3.0% or more, Si sharply increases the hardness of the ferrite phase and decreases the elongation, thus making it difficult to secure the austenite phase affecting the securement of the elongation. Also, an excessive amount of Si decreases the fluidity of slag in a steel making process and may be bonded to oxygen to form inclusions, thus decreasing corrosion resistance. Therefore, the content of Si may be determined in a range of 0.2-3.0%.

Nitrogen (N) is an element which greatly attributes to stabilization of the austenite phase and is one of elements which are concentrated on the austenite phase together with nickel (Ni) during an annealing treatment. Therefore, corrosion resistance and strength may be incidentally improved by increasing the content of nitrogen but solubility of nitrogen may be changed according to the content of manganese (Mn) added. So, it is necessary to adjust the content of nitrogen. In a range of Mn according to an exemplary embodiment of the present invention, when the content of nitrogen exceeds 0.3%, blow holes, pin holes or the like are generated during casting due to an excessive amount of nitrogen exceeding the solubility, so that surface defects may be caused in final products. Also, 0.150 or more of nitrogen should be added in order to secure corrosion resistance corresponding to that of 304 steel. When the content of nitrogen is relatively low, it may be difficult to secure a proper phase fraction. Therefore, the content of nitrogen may be determined within a range of 0.15-0.30%.

Manganese (Mn) is an element which serves as a deoxidizing agent, increases solubility of nitrogen, and forms austenite, and is added in replacement of expensive nickel (Ni). When the content of Mn exceeds 4%, it maybe difficult to secure corrosion resistance corresponding to the level of 304 steel. Also, when Mn is added in excess of 4%, solubility of nitrogen may be improved but corrosion resistance may be decreased because Mn bonds to sulfur (S) in steel to form MnS. When the content of Mn is less than 1%, it is difficult to secure an appropriate austenite phase fraction even by adjusting the content of an austenite-forming element such as Ni, Cu, N or the like and it fails to obtain a sufficient solubility of nitrogen at atmospheric pressure because the solubility of nitrogen added is low. Therefore, the content of Mn may be made in a range of 1-4%.

Chromium (Cr) is an element which stabilizes ferrite together with silicon (Si), plays a main role of securing a ferrite phase of two phase stainless steel, and is an essential element for securing corrosion resistance. The increase of content of Cr increases corrosion resistance but the content of relatively expensive nickel or other austenite-forming elements should be increased to maintain the phase fraction. Therefore, the content of Cr may be in a range of 19-23% so as to secure corrosion resistance as well as to maintain phase fraction.

Nickel (Ni) is an element which stabilizes austenite together with Mn, Cu, and N and plays a main role of securing austenite phase of duplex stainless steel. However, when nickel is excessively added, the austenite phase fraction increases to make it possible to secure an appropriate austenite fraction and manufacturing costs of products may be increased due to the use of relatively expensive nickel to make it difficult to secure competitiveness against 304 steel. Therefore, the balance of phase fraction may be sufficiently maintained by increasing other austenite-forming elements, e.g., Mn and N instead of decreasing the content of relatively expensive nickel as much as possible for cost savings. However, since it is possible to secure sufficient stabilization of austenite phase by suppressing the formation of plastic induced martensite generated in cold working with nickel, nickel may be added in an amount of 0.3% or more. Therefore, the content of nickel (Ni) may be made within a range of 0.3-2.5%.

When the content of copper (Cu) is 2.5% or more, it is difficult to process products due to hot shortness and thus the content of Cu may be set to a minimal amount in consideration of cost savings. However, copper may be added in an amount of 0.3% or more so as to secure sufficient stabilization of the austenite phase by suppressing the formation of the plastic induced martensite generated in cold working. Therefore, the content of Cu may be adjusted in a range of 0.3-2.5%.

MODE FOR CARRYING OUT THE PRESENT INVENTION

Hereinafter, the present invention will be described in more detail with examples thereof.

To identify the influence of nitrogen contained above the solubility limit in molten steel on strips, casting strips were manufactured by a casting method of Table 1 using molten steel having compositions listed in Table 1 and then were rolled to manufacture hot rolled strips. The content of each composition in Table 1 below indicates a value expressed in % by weight.

Examples (Comparative Example 2 and Examples 1-6) corresponding to rapid casting of Table 1 were conducted with 90 tons of molten steel using a twin-roll strip casting (i.e., rapid casting) method to manufacture casting strips having a width of 1,300 mm and a thickness of 4.0 mm, and directly after the casting, the casting strips were hot rolled at a high temperature to manufacture hot rolled strip coils having a thickness of 2.5 mm.

Meanwhile, an example (Comparative Example 1) indicated by an existing continuous casting method in Table 1 was cast using a continuous casting method.

Whether internal pores were generated or not was observed with respect to the hot rolled strips manufactured as above, and results are summarized in Table 1.

TABLE 1 Casting N Internal Item C Si Mn Cr: Ni Cu N method exhaust pore Compar- 0.05 1.35 2.8 20.3 1.06 1.0 0.23 Existing X ◯ ative continuous Example casting 1 Compar- 0.05 1.35 2.8 20.3 1.06 1.0 0.33 Rapid X ◯ tive casting Example 2 Example 0.045 1.08 3.02 19.63 0.98 0.98 0.272 Rapid ◯ X 1 casting Example 0.071 1.3 3.81 19.69 1.14 0.5 0.24 Rapid ◯ X 2 casting Example 0.051 1.28 3.07 20.02 1.0 0.503 0.24 Rapid ◯ X 3 casting Example 0.051 1.27 3.09 20.41 1.03 0.5 0.25 Rapid ◯ X 4 casting Example 0.02 1.21 2.63 20.53 0.85 0.793 0.22 Rapid ◯ X 5 casting Example 0.05 0.7 2.73 20.5 0.95 0.7 0.15 Rapid ◯ X 6 casting

As summarized in Table 1, the content of nitrogen (N) in Comparative Example 1 manufactured by an existing continuous casting method was 0.23%, but it could be understood that internal pores were generated in the hot rolled strip because nitrogen was not exhausted during the casting.

Since the nitrogen content of nitrogen (N) in Comparative Example 2 was 0.33% and high, it could be also understood that although the twin-roll strip casting method was applied, nitrogen was not sufficiently exhausted so that internal pores were generated in the hot rolled strip.

Meanwhile, the content of nitrogen (N) in Examples 1-6 corresponding to the present invention was 0.15-0.3%, and it could be understood that hot rolled strips were able to be cast without generation of internal pores by applying a twin-roll strip casting method of the present invention.

Meanwhile, a high temperature photograph of a hot rolled strip (Conventional Example) manufactured by an existing twin-roll strip casting method and a high temperature photograph of the hot rolled strip of Example 2 in Table 1 were observed and observation results were shown in FIGS. 5A and 5B, respectively. That is, FIG. 5A indicates the hot rolled strip of the conventional example in which solidification delay was generated in an edge section and FIG. 5B indicates the hot rolled strip of Example 2.

As shown in FIGS. 5A and 5B, in the case of the conventional example, a phenomenon in which a solidification shell is lifted off from the cast roll occurs, so that cooling ability of the cast roll in the edge section is lowered and thus the temperature of the edge section of the hot rolled strip (casting material) is elevated. Also, when such a phenomenon is severe, molten steel may flow without solidification. However, in the case of Example 1 of the present invention, since the temperature is uniform in the width direction, the hot rolled strip (casting material) may be manufactured without the generation of depressions.

Also, a high temperature photograph of a hot rolled strip (Conventional Example) manufactured by an existing twin-roll strip casting method and a high temperature photograph of the hot rolled strip of Example 2 in Table 1 were observed and observation results were shown in FIGS. 6A and 6B, respectively. That is, FIG. 6A shows a photograph of the conventional example in which an edge depression defect was generated, and FIG. 6B shows a photograph of the hot rolled strip (casting material) manufactured according to Example 2 of the present invention.

As shown in FIGS. 6A and 6B, it can be understood that in the case of the conventional example, a depression defect was generated due lack of gas exhaust or abrupt gas exhaust by deformation of the solidification shell. Such a depression defect may be generated in a vertical form or a horizontal form in a boundary between hill and valley. Also, since the depression may include micro-cracks that may be a cause of strip breakage, when such depression is generated in the edge section, cold rolling is conducted after the edge section is removed. However, it can be understood that Example 2 of the present invention has no depression defect.

Meanwhile, in order to manufacture hot rolled strips of Examples 2 and Comparative Example 1 in Table 1, hot annealing, cold rolling and cold annealing were conducted at a hot annealing temperature of 1,100° C. and a cold annealing temperature of 1,150° C.

After the cold annealing, microstructures of steel sheets were investigated and investigation results are shown FIGS. 7A and 7B.

FIG. 7A shows a photograph of a recrystallized structure of a cold annealed product manufactured by a continuous casting method and corresponding to Comparative Example 1 and FIG. 7B shows a microstructure of a cold annealed product manufactured by a twin-roll strip casting method according to Example 2 of the present invention.

In Comparative Example 1, grains elongated in the rolling direction were observed and ferrite and austenite were stacked and arranged. By virtue of such microstructure arrangement, the elongation was high when a tensile test was performed in the rolling direction but the elongation was low when the tensile test was performed in a direction perpendicular to the rolling direction. As an analysis result obtained by using an image analysis tool, the grains elongated in the rolling direction had an average length ranging from 9 to 10 μm and an average diameter of about 5 μm.

In Example 2, it could be confirmed that microstructures were randomly arranged without having a specific orientation and the plastic anisotropy was minimized due to the microstructures. Also, in the case of cold annealing, a necking-down width was shown to be 10 mm or less, which was as good as general 304 series stainless steel. As an analysis result obtained by using an image analysis tool in order to check the distribution and size of recrystallized grains, the grains elongated in the rolling direction had an average length ranging from about 4 to 9 μm and an average diameter of about 4 μm.

Furthermore, hot rolled strips of Examples 1 to 6 in Table 1 were subjected to hot annealing, cold rolling, and cold annealing, and then the elongations and yield strengths of the hot annealed product and the cold annealed product were respectively measured. As a result, the elongation of the cold annealed product was 30-55%, which was higher than that of the hot rolled product by about 5%. The yield strength of the cold annealed product was 320-680 MPa, which was slightly lower than that of the hot rolled product.

Since cast rolls in which a hill area ratio of edge sections is higher than that of center sections are used, a twin-roll strip caster capable of a thin stainless steel sheet with improved quality, a method for manufacturing a thin duplex stainless steel sheet using the same, and a thin duplex stainless steel sheet may be provided.

Various and advantageous advantages and effects of the present invention are not limited to the above description and will be more easily understood through description of exemplary embodiments of the present invention.

While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit of the present invention as defined by the appended claims. 

What is claimed is:
 1. A twin-roll strip caster comprising: a pair of casting rolls rotating in opposite directions; an edge dam installed such that a molten steel pool is formed on respective sides of the pair of casting rolls; and a meniscus shield provided to cover the molten steel pool such that a contact between the molten steel pool and air is blocked, wherein hills and valleys are alternately arranged on surfaces of the pair of cast rolls in circumferential directions thereof and a hill area ratio (an area ratio of hills) in edge sections thereof is higher than that in centers sections thereof.
 2. The twin-roll strip caster of claim 1, wherein the hill area ratio is constant in the center section and is continuously increased in the edge section in a direction away from a boundary between the edge section and the center section.
 3. The twin-roll strip caster of claim 1, wherein the hill area ratio in the center section is in a range of about 10-40% and the hill area ratio in the edge section increases up to 70%.
 4. The twin-roll strip caster of claim 1, wherein the edge sections have a width of 50-200 mm from one ends of the pair of casting rolls.
 5. The twin-roll strip caster of claim 1, wherein a gas discharge index G of the center section has a value of 80-130, the gas discharge index G of the edge section continuously decreases to a range of at least 50-70 from a boundary of the center section, and G=width (W) of valley depth (d)/pitch (p).
 6. A method for manufacturing a thin duplex stainless steel sheet, the method comprising: forming a cast strip by pouring molten steel between a pair of cast rolls rotating in opposite directions; and manufacturing a hot rolled strip by rolling the cast strip in a rolling mill, wherein hills and valleys are alternately arranged on surfaces of the pair of cast rolls in circumferential directions thereof and a hill area ratio (an area ratio of hills) in edge sections thereof is higher than that in centers sections thereof.
 7. The method of claim 6, wherein the reduction ratio is in a range of 15-60%.
 8. The method of claim 6, further comprising annealing the hot rolled strip, wherein the annealing temperature is in a range of 1,000-1,250° C.
 9. A thin duplex stainless steel sheet manufactured by the method as set forth in claim
 6. 10. The duplex stainless steel sheet of claim 9, wherein the thin duplex stainless steel sheet comprises, by weight: 0.1% or less carbon (C) (exclusive of 0%), 0.2-3.0% silicon (Si), 1.0-4.0% manganese (Mn), 19.0-23.0% chromium (Cr), 0.3-2.5% nickel (Ni), 0.15-0.3% nitrogen (N), 0.3-2.5% copper (Cu), a balance of iron (Fe), and inevitable impurities.
 11. The duplex stainless steel sheet of claim 9, wherein the thin duplex stainless steel sheet may has an elongation of 25-55% in a direction perpendicular to the rolling direction and a yield strength of 350-700 MPa.
 12. The duplex stainless steel sheet of claim 9, wherein the thin duplex stainless steel sheet has a recrystallized grain size of about 4-9 μm and a necking-down width of 10 mm or less. 